FR2760398A1 - METHOD FOR PRODUCING PRECISION HOLLOW PIECES OF COMPOSITE MATERIAL - Google Patents

Abstract

The invention proposes a method for producing hollow parts (1) in composite material comprising reinforcing fibers embedded in a matrix of hot-polymerized organic resin. Such a method is remarkable in that at least one layer of reinforcing fibers preimpregnated with resin (15, 16) is draped around the cores (10) of elastomer-silicone, and in that the assembly is then. molded simultaneously with centrifugal compression and centripetal compression obtained jointly by the thermal expansion of the cores (10) and by the bringing together of the walls (22, 27) of the mold (20).

Description

METHOD FOR PRODUCING PRECISION HOLLOW PIECES

COMPOSITE MATERIAL

  The invention relates to processes for producing hollow parts made of laminated composite material comprising reinforcing fibers embedded in a matrix of polymerized organic resin, and more particularly to processes suitable for producing parts with high characteristics in terms of

  resistance, precision and temperature resistance.

  The laminated composite materials comprising reinforcing fibers embedded in a polymerized resin matrix are particularly appreciated in aeronautics for their excellent strength / mass ratio and tend when possible to replace metal alloys, particularly in the case of parts composed of thin walls obtained

  traditionally by foundry or boilermaking.

  It is sought thereby to produce turbomachine parts and in particular aircraft turbine engines, for example low pressure compressor housing arms or low pressure compressor hollow blades which are composed of thin walls forming and surrounding through cavities, it is to say not completely closed. These parts must be monobloc, to avoid assembly areas generating weaknesses. These parts must also be precise with a good surface finish to avoid reworking. These parts must withstand high temperatures and must have a cost price comparable to or lower than the parts

metallic equivalent.

  The "resin transfer molding" or RTM process consists in arranging the reinforcing fibers in a mold in the form of the finished part, injecting very liquid resin under pressure into the mold and polymerizing the resin kept under pressure. This process makes it possible to obtain precise and resistant parts with very varied shapes. However, the resins employed do not have a good temperature resistance, which limits the use of the

  process to parts to stay cold.

  Resins resistant to higher temperatures do not have sufficient fluidity before polymerization. De facto, it is necessary to make hollow parts laminated with these resins: - prepreg plies of fabrics or fibers with the resin, - to constitute a nucleus possibly capable of being destroyed, - to surround the nucleus by an inflatable bladder made of elastomer - drape, that is to say arrange around the core + bladder layers of fabrics or pre-impregnated fibers, thus constituting the composite material, - put the whole core + bladder + composite material in a mold to the shape outer part of the finished part, - inflate the bladder, - polymerize the resin, - deflate the bladder and unmold, - remove or destroy the core,

- remove the bladder.

  In this method, inflation of the bladder simultaneously: press the composite material against the wall of the mold; - compress and flow the composite material to reduce the porosities from the air bubbles trapped between the fiber webs as well as gaseous emissions of the resin during the polymerization, drive off the excess resin and thereby increase the fiber density. A compression of

  % of the wall thickness is usually done.

  It is understood that with such a method, only the surface of the

  piece in contact with the wall of the mold is precise and smooth.

  The surface in contact with the bladder is on the contrary irregular and rough and follows the inevitable heterogeneities of the draping of the reinforcing fibers. Reciprocally, one could imagine compressing the composite material on the core, but this solution would cause an unacceptable folding of the reinforcing fibers, this folding reducing

the resistance of the piece.

  A first problem is therefore to obtain, from fibers or pre-impregnated fabrics, hollow pieces of variable shape whose inner and outer surfaces are

  precise and smooth, without wrinkling of the reinforcing fibers.

  When the cavities open to the outside through openings too small, which is often the case, the nuclei can be eliminated only by the destruction of the material that composes them. Marketable to make moldable materials to the desired shape which can then be dissolved with water or with a solvent after the piece has been molded. Such cores, however, are not suitable in the present case, because obtaining precise inner surfaces would require centripetal compression of the composite material on the core with the disadvantages described above. A second problem is therefore to eliminate the

  cores after molding the piece.

  A brake on the use of such composite materials is the high production cost of the parts compared to equivalent parts of metal alloys. This high cost comes in particular from the many manipulations required for manufacturing. It is therefore appropriate not to increase

  the complexity of the manufacturing process.

  The invention proposes a process for manufacturing hollow parts made of laminated composite material made from reinforcing fibers pre-impregnated with hot-polymerizable organic resin, said hollow parts comprising at least one cavity, said process comprising the following essential operations: a) production of cores in the shape of the cavities of the part, b) draping of the preimpregnated fiber around the cores, c) arrangement of the whole cores + fiber prepreg in a mold,

  d) hot polymerization, demolding and removal of the cores.

  Such a method is remarkable in that: a) The compression of the composite material is effected by bringing the walls of the mold closer to the cores, so as to simultaneously form the internal and external surfaces of the part and cause the creep of a sufficient quantity of resin during the polymerization b) The cores are thermally expandable silicone elastomer to stretch the reinforcing fibers during the

  polymerization and avoid their wrinkling.

  This solution is made possible by the very high coefficient of thermal expansion of the silicone elastomer. The elastomers of the silicone family are usually used as seals for temperatures that can be mounted according to the case and the service life of the packing at temperatures of the order of 250 C. The process that is the subject of this invention exploits a particular characteristic of these materials, namely a very high coefficient of thermal expansion close to

400.10-6 / C.

  The cores of the invention constituting solid bodies, unlike assemblies consisting of a core and an inflatable bladder of the prior art, which allows them to print their shape and surface condition to the composite material.

course of polymerization.

  The thermal expansion of the silicone elastomer cores also has the side effect of pushing the composite material against the walls of the mold, which causes a limited centrifugal compression of said composite material. This makes it possible to reduce the centripetal compression effected by bringing the walls of the mold closer together, and thus to reduce the

  deformations inflicted on the fiber during molding.

  If T1 is called the transition temperature of the resin from the pasty state to the solid state, the skilled person will give the core at this temperature T1, or at a temperature slightly lower than T1, the shape and dimensions of the cavity to be obtained, possibly corrected for the thermal expansion of the polymerized composite which remains very low, of the order of 1.10-6 / C. This has the effect of giving said cavities the

  dimensions required at the beginning of the hardening of the resin.

  The dimensions of the cold core will then be calculated by applying to the hot dimensions a coefficient corresponding to the thermal expansion of the silicone elastomer between the temperature T1 and the ambient temperature, the elastomer being itself usually obtained by

cold polymerization.

  The hardness of the elastomer is not critical, the person skilled in the art merely choosing an elastomer of sufficient hardness, at least equal to 30 shore A, for the possible deformations of the core to remain compatible with the precision required of the piece. If necessary, it is possible to increase the hardness of an elastomer by a load

  microbeads, for example glass.

  Advantageously, a silicone elastomer whose chipping temperature T2 is lower than the temperature will be chosen.

  T3 curing by complete polymerization of the resin.

  This has the effect of disintegrating the silicone elastomer core during molding under the effect of the temperature at which is carried the composite material during polymerization. The material constituting the core can then be easily removed from the cavity after molding by simply washing with water, or even scraping or brushing it.

which solves the second problem.

  In a preferred embodiment, a silicone elastomer will be chosen, the disintegration temperature T2 being between T1 and T3. This arrangement has the effect of maintaining the cores in the solid state until the cavities have been formed to the required dimensions, and then to break up the cores while the composite material is itself in the solid state, so as not to risk

to crack it.

  Advantageously also the cores will be strengthened by bars of stronger materials, for example metal alloy, to prevent the possible deformation of the cores during draping or molding. Advantageously finally, the reinforcing bars will overflow the core and will support in the mold to improve the accuracy of the positioning of the cores in the mold and by repercussion the accuracy of the positioning of the cavities of the room. The invention will be better understood and the advantages it affords will appear more clearly in view of an example

  Detailed embodiment and accompanying figures.

  FIG. 1 illustrates, in cross-section, a turbine engine low-pressure compressor casing arm for an aircraft being molded. For the sake of clarity, the thicknesses of the arm and the layers of fibers have been increased. FIG. 2 illustrates, in a partial longitudinal section along AA, the end of this same housing arm in FIG.

molding course.

  Referring firstly to FIG. 1, the piece 1 is thin and elongated and comprises two lateral parts or thin sides 2, 3 which meet at the rear end 4 as well as at the front end 5. In the middle of the piece 1, a rib 6 also connects the sides 2 and 3 between them to increase the rigidity and also increase the rigidity of the part 1. The sidewalls 2, 3 associated with the rib 6 define two elongated cavities 7. The inner surfaces of the part 1 forming the cavities 7 and the outer surfaces of said part 1 will be referenced 8. During the molding, the cavities 7 are each formed by a core 10 made of silicone elastomer. These cores 10 being long and thin are each stiffened by a metal bar 12 crossing them in the longitudinal direction, said bars having a very flat rectangular section whose edges 12a are rounded. The silicone core 10 is obtained by extrusion with a cavity 13 in the shape of the bar 11, followed by

  of a section at the length denoted L in FIG.

  The rod 11 is then introduced into the cavity 13 of the core, which presents no difficulty because of the even small elasticity of the core 10. The cores 10 are then draped with one or more layers of resin prepreg fibers 15. then arranges the cores taped 10 + 15 side by side and then draping the assembly by one or more layers of preimpregnated fibers of

  resin 16 so as to constitute the piece.

  The assembly 10 + 15 + 16 is then placed in the female part 21 of a mold 20, the bottom wall 22 having the shape of the outer surface of the sidewall 2 of the part 1, the wall of the bottom 22 being adjacent to two sidewalls 23a, 23b, said sidewalls 23a, 23b being parallel and extending by respectively flared sidewalls 24a, 24b adjacent to a bearing surface 25a, 25b respectively. The mold 20 also comprises a male part or punch 26 whose end 27 is a wall in the shape of the outer surface of the sidewall 3 of the part 1, this wall 27 being adjacent to two side walls 28a, 28b of complementary shape to the 23a, 23b walls, the punch 26 being slidably mounted with a clearance reduced by its walls 28a, 28b between the walls 23a, 23b which provide guidance. The side walls 28a, 28b are adjacent to bearing surfaces respectively 29a, 29b which come against

  the bearing surfaces 25a, 25b of the female part 21.

  The assembly is surrounded by a layer of felt 30 and a flexible envelope or sealed bag 31 connected by a nozzle 32 to a source of air depression 33. The whole is arranged between the two plates 34, 35 of an oven press no

represent.

  Referring now to FIG. 2, the ends 12b of the bars 11 protrude from each side of the core 10 and are taken between two mores 40, 41 coming into contact with one another via a separation surface 42, said mores being assembled by screws 43. The blocks 40, 41 are positioned in a cavity 44 of the mold 20 opening outwards, said cavity 44 having a shape complementary to the jaws 40, 41 with a clearance e of about 0.2 mm . The interior of the cavity 44 has two shoulders 45 for positioning the jaws longitudinally 40, 41. The opening portion of the cavity 44 is filled by a slot buffer 46. It is understood that the game e communicates the mass of material composite 15, 16 with the felt 46 and allows to evacuate the excess resin while maintaining sufficient internal pressure. The other end of the assembly not shown is symmetrical with the end shown in FIG. 2. The distance between the pair of jaws 40, 41 and the pair of mores not shown at the other end is equal to

at the length L of the core 10.

  In this case, the cores 10 are made of silicone elastomer of hardness 70 Shore A with a T2C fragmentation temperature of 290 ° C. and a coefficient of thermal expansion equal to 400 × 10 -6 / C. In this example, carbon fibers are used. whose coefficients of thermal expansion are substantially zero up to 300 C. These fibers consist of fabrics and prepregs of "PMR 15" heat-curable resin, "PMR 15" being a trademark of the company FIBERITE-USA. This resin has a transition temperature T1 of the pasty state in solid state equal to 280 C and a temperature T3 of hardening of

the resin equal to 320 C.

  The manufacturing process of the piece 1 comprises the following essential operations: a) constitution of the cores 10 by extrusion and cutting to the length L, b) introduction of a metal bar 11 in each core, the ends 12b of the bars 11 overflowing with each side of the core 10, c) draping at least one layer 15 of resin prepreg fibers around the cores 10, d) seizing and joining the sets of bars 11 + cores 10 + pre-impregnated layers 15 between the mores 40, 41 by a end 12b of the bars 11 and clamping the mores 40, 41 by the screws 43 e) repetition of the operation with a second pair of mores 40, 41 at the other end 12b of these same bars 11, f) draping the assembly thus obtained by at least one layer 16 of resin preimpregnated fibers, g) placing the assembly in the female portion 21 of the mold 20, and closure of the mold 20 with the male portion 26, h) setting up buffers of felt 46 at the entrance of cavit 44 of the mold 20, placed successively around the mold 20 of the felt layer 30 and the sealed bag 31 with the nozzle 32, and arrangement of the assembly between the

  trays 34, 35 of the oven-press not shown.

  The thermal cycle is that of polymerization of the resin.

  The rise in temperature is accompanied by the expansion of the cores 10 and the approximation of the walls 22 and 27 of the mold 20 between them and with respect to the cores 10 which causes the compression of the layers 15, 16 of preimpregnated fiber. The walls 22 and 27 are brought together by sliding the punch 26 between the walls 23a, 23b of the female part 21 of the mold 20, until the bearing surfaces 25 and 29 come into contact with each other. on the other, which accurately positions the walls 22 and 27 of the mold between them and with respect to the cores 10, and guarantees the accuracy and quality of the outer surfaces of the two sides 2 and 3, as well as the accuracy of the dimensions and the positioning of the cavities 7 with respect to the sidewalls 2 and 3. Sliding takes place under the combined effect of the pressure exerted by the trays 34, 35 of the press furnace and the pressure

  atmospheric exerted on the bag 31 put in depression.

  Under the effect of the pressure, the excess of resin is evacuated in the cavities 44 around the mores 40, 41 by the clearance e which is left between said mores 40, 41 and the walls of said cavities 44, the resin coming from accumulate in the felt pads 46. If necessary, the skilled person may also provide additional cavities not shown in the

  mold to receive the excess resin.

  The still liquid resin emits under the effect of the polymerization of the gases which must imperatively be

  evacuated to remove the porosities of the composite material.

  This evacuation is facilitated by the depression of the bag 31 from the nozzle 32, this depression being transmitted to the resin through the felt disposed between the bag 31 and the mold 20 between the walls.

  23ab and 28ab, and by the felt pads 46 and the game e.

  Advantageously also the mores 40, 41 can be pierced by a plurality of conical holes 47 placing the composite material in communication with the felt pads 46, in order to

  facilitate the evacuation of the resin by exceeding and gases.

  The taper of the holes 47 facilitates the removal of the resin

hardened after molding.

  When the temperature T of the composite material has reached the transition temperature T1 = 280 ° C between the pasty state and the solid state, the elastomer-silicone cores 10 have reached by thermal expansion the dimensions of the cavities 7 to obtain this which guarantees the accuracy. At T> Tl, the solidified composite material held by the walls 22, 27 of the mold 20 opposes the expansion of the cores 10 whose material begins to crosslink. When the temperature T has reached the temperature T2 = 290 of fragmentation of the elastomer, the cores 10 begin to disintegrate and no longer exert pressure on the composite material. The polymerization can then continue until the

  curing temperature of the resin T3 = 320 C.

  The assembly is then demolded hot so that the mold 20 by contracting with the decrease in temperature does not crush the composite material which is now solidified and hardened. Then, the resin masses are broken and the excess is removed, the jaws 40, 41 are removed, the bars 11 are removed and the material of the fragmented cores 10 is mechanically removed inside the cavities 7 by any

  non-abrasive mechanical means: scraping, blowing, washing.

  With such a method, a turbomachine casing arm having a length L = 400 mm and a width of 12 mm, the flanks 2, 3 having a thickness of two millimeters, was obtained rough molding with a geometric accuracy of 0, 2 mm and perfectly smooth interior and exterior surfaces, the mechanical recoveries being limited to the debugging of the resin hardened along the lines of evacuation of said resin exceeding, as well as to the holes of

  fastening to the ends of the piece 1.

  The porosity of the composite material has been reduced to 2% by virtue of the combination of centripetal and centrifugal compression of said composite material and to the depression of the bag 31. Thanks to the resin used, the piece 1 can be used at a temperature of 280 C service and can withstand

  temporarily a temperature up to 325 C.

  The invention is not limited to the embodiment which has just been given, but on the contrary covers variants that could be made to him at the level of the piece to obtain and means to implement: obviously, the cores may be of varying number and shape depending on the part to be obtained, and the holes through which the cavities open may be reduced in size relative to said cavities. The use of the bars 11 is not necessary when the cores 10 are short and therefore not exposed to flexion. A harder elastomer will then be used to prevent any deformation of said cores. In the case where the part 1 has only one cavity, it is possible to use only one layer of resin prepreg fiber. In the case of flattened parts, the centripetal compression will be made on the flanks 2, 3 of the

room 1.

  In this example, the walls of the cavities are formed of parallel geometric lines, which made it possible to obtain cores 10 directly by extrusion and cut to the length L. In the case of cavities of any shape, for example in a cask, the core 10 will be obtained by molding. It will be noted that the metal bars 11 may be omitted in the case of the cores 10 of compacted form, the core 10 being able to be stiffened by the choice of an elastomer having a higher Shore A hardness, or by inclusion of microbeads in the elastomer. The holes communicating the cavities with the outside may be reduced in size, since the cores crumble during the thermal polymerization cycle, which then simplifies the removal of the

material composing them.

  The joint use of the vacuum bag 31 and the press furnace makes it possible to reach the pressure of 8 bars required.

  for compression of the resin used in this example.

  It is also possible to use an ordinary press and a heated mold, for example with electrical resistors. In the case where the resin requires only a compression less than one bar, this compression can be obtained without press under

  the only effect of atmospheric pressure.

Claims (7)

  1. A method of producing hollow parts made of laminated composite material comprising reinforcing fibers embedded in a hot polymerized resin matrix, said hollow part (1) comprising at least one cavity (7), said method comprising the following essential operations: a) forming cores (10) in the form of the cavities (7) of the part (1), b) draping the cores (10) by at least one layer of reinforcing fibers impregnated with heat-curable resin (15, 16 ), to constitute the composite material, c) arrangement of the core assembly (10) + pre-impregnated fibers (15, 16) in a mold (20), d) compression, polymerization, demolding of the workpiece (1) and removal of the cores (10), characterized in that: a) the compression is effected by bringing the walls (22, 27) of the mold (20) closer to the cores (10, to simultaneously form the inner (8) and outer (8) surfaces ( 9) and cause sufficient creep of the resin during the polymerization, b) the cores (10) are heat expandable silicone elastomer, in order to tension the reinforcing fibers during the polymerization.   2. Method according to claim 1, characterized in that at the transition temperature T1 of the pasty state in the solid state of the resin, the cores (10) of silicone elastomer have the dimensions of the cavities (7) of the piece (1) at this temperature T1, in order to give said cavities (7) the   dimensions required at the beginning of the hardening of the resin.   3. Method according to claim 1 or 2, characterized in that the silicone elastomer of the cores (10) has a crumbling temperature T2 lower than the temperature T3 of complete polymerization of the resin in order to disaggregate the   cores (10) during molding of the workpiece.   4. Method according to claim 3, characterized in that the elastomersilicone has a crumbling temperature between the transition temperature T1 of the pasty state in the solid state of the resin and the temperature T3 of complete polymerization of the resin, in order to maintain the cores (10) in the solid state until the cavities have been formed to the required dimensions, and in order then to disintegrate said cores (10) while the composite material is in the solid state.   5. Method according to any one of claims 1 to 4,   characterized in that the cores (10) are reinforced by bars (11) of stronger material, to prevent deformation of said cores (10) during draping of the   composite material (15, 16) or molding.   6. Method according to claim 5, characterized in that the reinforcement bars (11) protrude from the cores (10) and bear in the mold (20) to ensure the accuracy of the positioning of said cores (10) in said mold (20).   7. Method according to any one of claims 1 to 6,   said method employing at least two cores (10), characterized in that: a) each core (10) is covered separately with at least one layer of composite material (15), b) the covered nuclei (10) are then joined together; +15) and the resulting assembly is covered by at least one layer of material composite (16).